1. Field of the Invention
The present invention relates generally to image forming equipment and is particularly directed to electrophotographic (EP) printers of the type which use fusers to fix toner to print media, such as paper sheets. The invention is specifically disclosed as a belt fuser assembly for a color EP printer that quickly warms up the fusing belt before allowing a first sheet of print media to run through the fuser assembly. The warm-up cycle includes: (1) a preheat mode, (2) a first portion of a belt temperature warm-up mode, (3) a second portion of the belt temperature warm-up mode, and (4) a tight belt temperature control mode. Certain parameters are sensed or calculated before moving from one mode to the next.
In one embodiment, the preheat mode operates the fuser's heater at less than full power, and also prevents the fuser belt from rotating. During the first portion of the belt temperature warm-up mode, the heater operates at full power, and the fuser belt begins rotating, but not at its full speed. During the second portion of the belt temperature warm-up mode, the heater operates at less than full power (according to a temperature setpoint algorithm), and the fuser belt begins rotating at its full speed. During the tight belt temperature control mode, the fuser belt begins running at its “printing speed” if a print job is “ready,” and the belt temperature is controlled rather tightly to remain within its proper fusing temperature tolerance.
The temperature setpoint used for controlling the heater's temperature is varied, according to the control mode. During the preheat mode, the setpoint is started at one value (e.g., 120° C.), and during the full power portion of the belt temperature warm-up mode the setpoint is raised to a greater value (e.g., 240° C.), to force the fuser system to quickly increase in temperature. The temperature of the belt is not directly sensed in the illustrated embodiment, and it is instead inferred from a temperature sensor at the heater itself, and from calculating the AC line voltage that is powering the fuser's heater element by how quickly the heater rises in temperature during the early steps of the warm-up process.
2. Description of the Related Art
Two of the most important issues when using a belt fuser for a color laser printer are the possibility of the heater element cracking and also the amount of the warm-up time required before the first print can be made. To reduce the fuser's warm-up time, the heating power needs to be as high as possible. However, excessive heating power can crack the heating element. One way to prevent the heater from cracking is to limit the heater temperature ramping rate during the fuser warm-up, and that is used in many monochrome belt fusers. Unfortunately by limiting the temperature ramp-up rate, this also increases the fuser's warm-up time.
The fusing belt of color electrophotographic (EP) fusers typically has a larger thermal mass as compared to fusing belts used for monochrome EP printers. In addition, some of the newer color EP printers have a larger backup roll. All of these factors make the color belt fuser warm-up much slower than that of a monochrome belt fuser. To decrease the amount of time required before the first printed copy is made, a new fuser warm-up algorithm is needed that can minimize the fuser warm-up time while not cracking the heater element.